I can only post my experience with IRC600 and Flysights Black Pearl monitor. It works very well for me. I get over 1km clear video even on a turn. I know I could get much further but I have no need, its photography I'm doing, and if honest, I am little bit chicken. I did get better antennae though.
I think the confusion comes in as there are several different, models, sub-models and unbranded ones and they perform differently and maybe even country specific models. I got mine from a genuine UK dealer and not banggood or whatever. Its the 32 channel RC801 with SMA (hole)
Bottom line: I'm a happy chappy
Sent from my Galaxy Note 8